True nullability schema work in progress

Author: benjie

spec/Section 2 -- Language.md

@@ -1238,6 +1238,12 @@ NonNullType :
 
 - NamedType !
 - ListType !
+- NonNullErrorBoundaryType
+
+NonNullErrorBoundaryType :
+
+- NamedType ! ?
+- ListType ! ?
 
 GraphQL describes the types of data expected by arguments and variables. Input
 types may be lists of another input type, or a non-null variant of any other

spec/Section 3 -- Type System.md

@@ -1787,11 +1787,16 @@ Following are examples of input coercion with various list types and values:
 ## Non-Null
 
 By default, all types in GraphQL are nullable; the {null} value is a valid
-response for all of the above types. To declare a type that disallows null, the
-GraphQL Non-Null type can be used. This type wraps an underlying type, and this
-type acts identically to that wrapped type, with the exception that {null} is
-not a valid response for the wrapping type. A trailing exclamation mark is used
-to denote a field that uses a Non-Null type like this: `name: String!`.
+response for all of the above types. To declare a type that is not expected to
+be null, the GraphQL Non-Null type can be used. This type wraps an underlying
+type, and this type acts identically to that wrapped type, with the exception
+that {null} is not an expected response for the wrapping type. A Non-Null type
+may be an error-boundary in which case the response is allowed to be {null}, but
+only with an associated error in the errors array; otherwise {null} is not a
+valid value. A trailing exclamation mark is used to denote a field that uses a
+Non-Null type like this: `name: String!`, and an additional `?` is used to
+indicate that this Non-Null type is an error boundary, like this:
+`name: String!?`.
 
 **Nullable vs. Optional**
 
@@ -1810,8 +1815,10 @@ always optional and non-null types are always required.
 In all of the above result coercions, {null} was considered a valid value. To
 coerce the result of a Non-Null type, the coercion of the wrapped type should be
 performed. If that result was not {null}, then the result of coercing the
-Non-Null type is that result. If that result was {null}, then a _field error_
-must be raised.
+Non-Null type is that result. If that result was {null}, then: if the Non-Null
+is an error boundary then the result of coercing the Non-Null type is {null} but
+a _field error_ must be collected; otherwise if the Non-Null is not an error
+boundary then a _field error_ must be raised.
 
 Note: When a _field error_ is raised on a non-null value, the error propagates
 to the parent field. For more information on this process, see

spec/Section 4 -- Introspection.md

@@ -151,6 +151,8 @@ type __Type {
   ofType: __Type
   # may be non-null for custom SCALAR, otherwise null.
   specifiedByURL: String
+  # must be non-null for NON_NULL, otherwise null.
+  isErrorBoundary: Boolean
 }
 
 enum __TypeKind {
@@ -168,7 +170,7 @@ type __Field {
   name: String!
   description: String
   args(includeDeprecated: Boolean = false): [__InputValue!]!
-  type: __Type!
+  type(includeErrorBoundaries: Boolean! = false): __Type!
   isDeprecated: Boolean!
   deprecationReason: String
 }
@@ -395,8 +397,13 @@ Fields\:
 GraphQL types are nullable. The value {null} is a valid response for field type.
 
 A Non-Null type is a type modifier: it wraps another type instance in the
-`ofType` field. Non-null types do not allow {null} as a response, and indicate
-required inputs for arguments and input object fields.
+`ofType` field. A Non-Null type may opt to be an error boundary, in which case
+it indicates that the value is not expected to be null: it will only be null in
+the case of an error (either it's own, or one of its children). For error
+boundary non-null types, the `null` value will not "bubble". Error boundary
+non-null types are only allowed in output positions. Non-null types that are not
+error boundaries do not allow {null} as a response, and indicate required inputs
+for arguments and input object fields.
 
 The modified type in the `ofType` field may itself be a modified List type,
 allowing the representation of Non-Null of Lists. However it must not be a
@@ -406,6 +413,8 @@ Fields\:
 
 - `kind` must return `__TypeKind.NON_NULL`.
 - `ofType` must return a type of any kind except Non-Null.
+- `isErrorBoundary` must return true if this Non-Null is an error boundary,
+  false otherwise.
 - All other fields must return {null}.
 
 ### The \_\_Field Type
@@ -422,6 +431,10 @@ Fields\:
     {true}, deprecated arguments are also returned.
 - `type` must return a `__Type` that represents the type of value returned by
   this field.
+  - Accepts the argument `includeErrorBoundaries` which defaults to {false}. If
+    {false} then the field will instead return a `__Type` that represents the
+    type of value returned by this field with all error-boundary non-null type
+    modifiers removed. This is for backwards compatibility.
 - `isDeprecated` returns {true} if this field should no longer be used,
   otherwise {false}.
 - `deprecationReason` optionally provides a reason why this field is deprecated.

spec/Section 6 -- Execution.md

@@ -674,7 +674,10 @@ CompleteValue(fieldType, fields, result, variableValues):
   - Let {innerType} be the inner type of {fieldType}.
   - Let {completedResult} be the result of calling {CompleteValue(innerType,
     fields, result, variableValues)}.
-  - If {completedResult} is {null}, raise a _field error_.
+  - If {completedResult} is {null}:
+    - If {fieldType} is an error boundary, collect a _field error_ and return
+      {null}.
+    - Otherwise, raise a _field error_.
   - Return {completedResult}.
 - If {result} is {null} (or another internal value similar to {null} such as
   {undefined}), return {null}.